Chemical vapour deposition (CVD) grown graphene presents an efficient synthetic route for large-scale graphene production, however, current transfer methods are focused on transferring graphene onto planar substrates. Here, we describe a method to uniformly transfer CVD-grown graphene onto 3D objects with high shape complexity. Due to its promises as a coating for medical applications, we investigated graphene transfer onto commercially available titanium implants with different geometries using a vacuum-assisted dry transfer method. A self-standing graphene-polymer tape is obtained by spin coating of a silicone polymer on the graphene, followed by the addition of a support layer, and the electrochemical delimitation of the growth substrate. The tape is then wrapped around the object and placed into a mould to conform to the surface. Using a pressure differential provided via partial vacuum, a uniform force is applied to the surface. Graphene characteristics are assessed by Raman spectroscopy and AFM. 74\% and 95\% coverage yields are obtained after a single transfer and double transfer respectively, with an observed increase in the number of graphene layers and an overall decrease in defect contributions. In vitro cell assays demonstrate that the coating is non-cytotoxic enabling further exploration of this method for medical applications.

Black phosphorus (BP) is a layered crystalline structure presenting a thickness-tunable direct bandgap and a high charge carrier mobility, with, therefore, enormous interest to photonics, optoelectronics and electronics. However, BP\&$\#$39;s high susceptibility to oxidation when exposed to ambient conditions is a critical challenge for its implementation into functional systems. Here, we investigate the degradation of BP flakes exposed to various environmental conditions by synchrotron infrared nanospectroscopy (SINS). As a near-field based technique, SINS provides sub-diffractional mid-infrared images and spectra from nano-sized domains. Supported by density functional theory (DFT) calculations, our SINS spectra reveal the formation of nanoscale POx domains, with x between 0.5 and 1, and a 100 meV red shift in the bandgap of flakes exposed to air for a few minutes. On the other hand, exposure to air for 24 h led to the preferential formation of H3PO4, with complete removal of the electronic transitions from the mid-infrared spectral window, while a long (1 month) exposure to low O-2 levels mainly led to the formation of P4O8 and P4O9 species. The SINS analysis allows correlating the morphology of oxidized samples to the oxide type, thus, contributing to a comprehensive characterization of the BP degradation process.

The physics of two-dimensional ( 2D) materials and heterostructures based on such crystals has been developing extremely fast. With these new materials, truly 2D physics has begun to appear ( for instance, the absence of long-range order, 2D excitons, commensurate-incommensurate transition, etc.). Novel heterostructure devices-such as tunneling transistors, resonant tunneling diodes, and light-emitting diodes-are also starting to emerge. Composed from individual 2D crystals, such devices use the properties of those materials to create functionalities that are not accessible in other heterostructures. Here we review the properties of novel 2D crystals and examine how their properties are used in new heterostructure devices.

Black phosphorus has recently emerged as a new layered crystal that, due to its peculiar and anisotropic crystalline and electronic band structures, may have important applications in electronics, optoelectronics and photonics. Despite the fact that the edges of layered crystals host a range of singular properties whose characterization and exploitation are of utmost importance for device development, the edges of black phosphorus remain poorly characterized. In this work, the atomic structure and behaviour of phonons near different black phosphorus edges are experimentally and theoretically studied using Raman spectroscopy and density functional theory calculations. Polarized Raman results show the appearance of new modes at the edges of the sample, and their spectra depend on the atomic structure of the edges (zigzag or armchair). Theoretical simulations confirm that the new modes are due to edge phonon states that are forbidden in the bulk, and originated from the lattice termination rearrangements.

Electromagnetic interaction between a sub-wavelength particle (the \"probe\") and a material surface (the \"sample\") is studied theoretically. The interaction is shown to be governed by a series of resonances corresponding to surface polariton modes localized near the probe. The resonance parameters depend on the dielectric function and geometry of the probe as well as on the surface reflectivity of the material. Calculation of such resonances is carried out for several types of axisymmetric probes: spherical, spheroidal, and pear-shaped. For spheroids, an efficient numerical method is developed, capable of handling cases of large or strongly momentum-dependent surface reflectivity. Application of the method to highly resonant materials, such as aluminum oxide (by itself or covered with graphene), reveals a rich structure of multi-peak spectra and nonmonotonic approach curves, i.e., the probe-sample distance dependence. These features also strongly depend on the probe shape and optical constants of the model. For less resonant materials such as silicon oxide, the dependence is weak, so that the spheroidal model is reliable. The calculations are done within the quasistatic approximation with radiative damping included perturbatively. (C) 2016 AIP Publishing LLC.

The interaction between two different materials can present novel phenomena that are quite different from the physical properties observed when each material stands alone. Strong electronic correlations, such as magnetism and superconductivity, can be produced as the result of enhanced Coulomb interactions between electrons. Two-dimensional materials are powerful candidates to search for the novel phenomena because of the easiness of arranging them and modifying their properties accordingly. In this work, we report magnetic effects in graphene, a prototypical non-magnetic two-dimensional semi-metal, in the proximity with sulfur, a diamagnetic insulator. In contrast to the well-defined metallic behaviour of clean graphene, an energy gap develops at the Fermi energy for the graphene/sulfur compound with decreasing temperature. This is accompanied by a steep increase of the resistance, a sign change of the slope in the magneto-resistance between high and low fields, and magnetic hysteresis. A possible origin of the observed electronic and magnetic responses is discussed in terms of the onset of low-temperature magnetic ordering. These results provide intriguing insights on the search for novel quantum phases in graphene-based compounds.

The relation between unusual Mexican-hat band dispersion, ferromagnetism, and ferroelasticity is investigated using a combination of analytical, first-principles, and phenomenological methods. The class of material with Mexican-hat band edge is studied using the alpha-SnO monolayer as a prototype. Such a band edge causes a van Hove singularity diverging with 1/root E, and a charge doping in these bands can lead to time-reversal symmetry breaking. Herein, we show that a material with Mexican-hat band dispersion, alpha-SnO, can be ferroelastic or paraelastic depending on the number of layers. Also, an unexpected multiferroic phase is obtained in a range of hole density for which the material presents ferromagnetism and ferroelasticity simultaneously.

The excitonic spectra of single-layer GeS and GeSe are predicted by ab initioGWand Bethe-Salpeter equation calculations. G(0)W(0) calculations for the band structures find a fundamental band gap of 2.85 eV for GeS and 1.70 eV for GeSe monolayer. However, excitons are tightly bound, specially in GeS at the Gamma point, where the quasiparticle interactions are so strong that they shift the Gamma exciton peak energy into the visible range and below the off-Gamma exciton peak. The lowest energy excitons in both materials are excited by light along the zigzag direction and have exciton binding energies of 1.05 and 0.4 eV, respectively; but despite the strong binding, the calculated binding energies are in agreement with a Mott-Wannier model.

Two-dimensional layered transition-metal dichalcogenides have attracted considerable interest for their unique layer-number-dependent properties. In particular, vertical integration of these two-dimensional crystals to form van der Waals heterostructures can open up a new dimension for the design of functional electronic and optoelectronic devices. Here we report the layer-number-dependent photocurrent generation in graphene/MoS2/graphene heterostructures by creating a device with two distinct regions containing one-layer and seven-layer MoS2 to exclude other extrinsic factors. Photoresponse studies reveal that photoresponsivity in one-layer MoS2 is surprisingly higher than that in seven-layer MoS2 by seven times. Spectral-dependent studies further show that the internal quantum efficiency in one-layer MoS2 can reach a maximum of 65\%, far higher than the 7\% in seven-layer MoS2. Our theoretical modelling shows that asymmetric potential barriers in the top and bottom interfaces of the graphene/one-layer MoS2/graphene heterojunction enable asymmetric carrier tunnelling, to generate usually high photoresponsivity in one-layer MoS2 device.

We demonstrate a straightforward and effective laser pruning approach to reduce multilayer black phosphorus (BP) to few-layer BP under ambient condition. Phosphorene oxides and suboxides are formed and the degree of laser-induced oxidation is controlled by the laser power. Since the band gaps of the phosphorene suboxide depend on the oxygen concentration, this simple technique is able to realize localized band gap engineering of the thin BP. Micropatterns of few-layer phosphorene suboxide flakes with unique optical and fluorescence properties are created. Remarkably, some of these suboxide flakes display long-term (up to 2 weeks) stability in ambient condition. Comparing against the optical properties predicted by first-principle calculations, we develop a \"calibration\" map in using focused laser power as a handle to tune the band gap of the

Black phosphorus has an orthorhombic layered structure with a layer-dependent direct band gap from monolayer to bulk, making this material an emerging material for photodetection. Inspired by this and the recent excitement over this material, we studied the optoelectronics characteristics of high-quality, few-layer black phosphorus-based photodetectors over a wide spectrum ranging from near-ultraviolet (UV) to near-infrared (NIR). It is demonstrated for the first time that black phosphorus can be configured as an excellent UV photodetector with a specific detectivity similar to 3 x 10(13) Jones. More critically, we found that the UV photoresponsivity can be significantly enhanced to similar to 9 x 10(4) A W-1 by applying a source-drain bias (V-SD) of 3 V, which is the highest ever measured in any 2D material and 10(7) times higher than the previously reported value for black phosphorus. We attribute such a colossal UV photoresponsivity to the resonant-interband transition between two specially nested valence and conduction bands. These nested bands provide an unusually high density of states for highly efficient UV absorption due to the singularity of their nature.

The stability of the surface of in situ cleaved black phosphorus crystals upon exposure to atmosphere is investigated with synchrotron-based photoelectron spectroscopy. After 2 days atmosphere exposure a stable subnanometer layer of primarily P2O5 forms at the surface. The work function increases by 0.1 eV from 3.9 eV for as-cleaved black phosphorus to 4.0 eV after formation of the 0.4 nm thick oxide, with phosphorus core levels shifting by {\textless}0.1 eV. The results indicate minimal charge transfer, suggesting that the oxide layer is suitable for passivation or as an interface layer for further dielectric deposition.

The optical response of phosphorene can be gradually changed by application of moderate uniaxial compression, as the material undergoes the transition into an indirect gap semiconductor and eventually into a semimetal. Strain tunes not only the gap between the valence band and conduction band local extrema but also the effective masses, and in consequence, the exciton anisotropy and binding strength. In this article, we consider from a theoretical point of view how the exciton stability and the resulting luminescence energy evolves under uniaxial strain. We find that the exciton binding energy can be as large as 0.87 eV in vacuum for 5\% transverse strain, placing it amongst the highest for two-dimensional materials. Further, the large shift of the luminescence peak and its linear dependence on strain suggest that it can be used to probe directly the strain state of single layers.

Heteroepitaxy of two-dimensional (2D) crystals, such as hexagonal boron nitride ({BN}) on graphene (G), can occur at the edge of an existing heterointerface. Understanding strain relaxation at such 2D laterally fused interface is useful in fabricating heterointerfaces with a high degree of atomic coherency and structural stability. We use in situ scanning tunneling microscopy to study the 2D heteroepitaxy of {BN} on graphene edges on a Ru(0001) surface with the aim of understanding the propagation of interfacial strain. We found that defect-free, pseudomorphic growth of {BN} on a graphene edge ?substrate? occurs only for a short distance ({\textless}1.29 nm) perpendicular to the interface, beyond which misfit zero-dimensional dislocations occur to reduce the elastic strain energy. Boundary states originating from a coherent zigzag-linked G/{BN} boundary are observed to greatly enhance the local conductivity, thus affording a new avenue to construct one-dimensional transport channels in G/{BN} hybrid interface.

Analysis of the strain-induced pseudomagnetic fields generated in graphene nanobulges under three different substrate scenarios shows that, in addition to the shape, the graphene-substrate interaction can crucially determine the overall distribution and magnitude of strain and those fields, in and outside the bulge region. We utilize a combination of classical molecular dynamics, continuum mechanics, and tight-binding electronic structure calculations as an unbiased means of studying pressure-induced deformations and the resulting pseudomagnetic field distribution in graphene nanobubbles of various geometries. The geometry is defined by inflating graphene against a rigid aperture of a specified shape in the substrate. The interplay among substrate aperture geometry, lattice orientation, internal gas pressure, and substrate type is analyzed in view of the prospect of using strain-engineered graphene nanostructures capable of confining and/or guiding electrons at low energies. Except in highly anisotropic geometries, the magnitude of the pseudomagnetic field is generally significant only near the boundaries of the aperture and rapidly decays towards the center of the bubble because under gas pressure at the scales considered here there is considerable bending at the edges and the central region of the nanobubble displays nearly isotropic strain. When the deflection conditions lead to sharp bends at the edges of the bubble, curvature and the tilting of the pz orbitals cannot be ignored and contributes substantially to the total field. The strong and localized nature of the pseudomagnetic field at the boundaries and its polarity-changing profile can be exploited as a means of trapping electrons inside the bubble region or of guiding them in channellike geometries defined by nanoblister edges. However, we establish that slippage of graphene against the substrate is an important factor in determining the degree of concentration of pseudomagnetic fields in or around the bulge since it can lead to considerable softening of the strain gradients there. The nature of the substrate emerges thus as a decisive factor determining the effectiveness of nanoscale pseudomagnetic field tailoring in graphene.

The development of spintronics devices relies on efficient generation of spin-polarized currents and their electric-field-controlled manipulation. While observation of exceptionally long spin relaxation lengths makes graphene an intriguing material for spintronics studies, electric field modulation of spin currents is almost impossible due to negligible intrinsic spin\–orbit coupling of graphene. In this work, we create an artificial interface between monolayer graphene and few-layer semiconducting ​tungsten disulphide. In these devices, we observe that graphene acquires spin\–orbit coupling up to 17 {meV}, three orders of magnitude higher than its intrinsic value, without modifying the structure of the graphene. The proximity spin\–orbit coupling leads to the spin Hall effect even at room temperature, and opens the door to spin field effect transistors. We show that intrinsic defects in ​tungsten disulphide play an important role in this proximity effect and that graphene can act as a probe to detect defects in semiconducting surfaces.

Black phosphorus thin films might offer attractive alternatives to narrow-gap compound semiconductors for optoelectronics across mid- to near-infrared frequencies. In this work, we calculate the optical conductivity tensor of multilayer black phosphorus thin films using the Kubo formula within an effective low-energy Hamiltonian. The optical absorption spectra of multilayer black phosphorus are shown to vary sensitively with thickness, doping, and light polarization. In conjunction with experimental spectra obtained from infrared absorption spectroscopy, we also discuss the role of interband coupling and disorder on the observed anisotropic absorption spectra.